Can Exchange send outgoing mail over different IP addresses based upon primary SMTP address?

Like a lot of people, we have multiple domains that we accept email on.  We basically have two companies in the same building:  @company1.com and @company2.com.   Both use the same exchange server and it works just fine.  I set the users for copmany1 to have a primary SMTP address as that domain and the users for company2 to have a primary for the other domain....  so in other words when company1 users email outbound their email address shows as user@company1.com and when users from the second company email out their email shows as user@company2.com.

My problem is reverse DNS.  RDNS only allows one domain to be set per IP address.  Can Exchange send outgoing email over one IP address for one domain and another IP for the other?  I have 5 IP's from my ISP and I know how to translate outgoing mail to a specific IP, but I need to have a unique identifier from the inside traffic because currently I'm translating all traffic from my mail server's IP over port 25.......however that includes outgoing email from both domains.   Does that make sense?   Thanks so much!
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tphelps19IT ManagerAsked:
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Alan HardistyCo-OwnerCommented:
Having multiple domain on one server is not a problem and rDNS should also not be a problem as long as it is setup correctly.
You cannot route mail out of one server down different IP addresses unless you forward the mail to an internal smart-host that can then push the mail out of the relevant IP.
Exchange can only route mail via recipient domain, not sending domain, so on its own - that's not an option.
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tphelps19IT ManagerAuthor Commented:
How would rDNS not be a problem when you can only assign one domain per IP address?  Currently all our mail goes out over one IP address but we have two different domains set up for primary email.  It works for company1 but when an rDNS lookup is performed on email sent from company2, it returns an incorrect flag because it returns the domain for company1.  What am I missing?

I hear what you are saying about a smart host.  Is there any software you can recommend that I could route outgoing mail to that can split and push mail out via a relevant IP based upon that information?
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Alan HardistyCo-OwnerCommented:
I have a single IP and host multiple domains - I don't have issues sending out mail as any of those domains and there are no rDNS issues.
For rDNS to work, the IP address (e.g., 123.123.123.123) should resolve to something like mail.yourdomain.com and mail.yourdomain.com should resolve to the IP address 123.123.123.123.  That's it.
Do you have mail for the other domains delivered direct to your server or do you collect it via POP3?
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tphelps19IT ManagerAuthor Commented:
Ok maybe I have rDNS wrong in my head then.  When I set my rDNS info, what exactly am I supposed to give to my ISP?  Just any domain name that resolves to the same IP?
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Alan HardistyCo-OwnerCommented:
You need to give them a Fully Qualified Domain Name (FQDN), such as mail.xyz.com, but in DNS for domain xyz.com, mail.xyz.com should point to the same IP address that the ISP is giving you.
Drop me an email and I will email you with specifics if you give me your IP and domain name(s).
View my profile view details (click on my name to see my profile).  Send an email from a domain that does not have rDNS problems or my anti-spam software may reject you!!
 
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hwdinfotechCommented:
alanhardisty is 100% correct.
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Alan HardistyCo-OwnerCommented:
: )
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tphelps19IT ManagerAuthor Commented:
Ok, I see what you guys are saying.....and I'm making those changes today with my ISP.  What I don't understand though, is how does that prevent a spammer from doing anything?  All they have to do is set any domain name to resolve back to the IP they send from.  What does that do?
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Alan HardistyCo-OwnerCommented:
Spammers don't generally use fixed IP Addresses so therefore they don't have rDNS to worry about.
Mail server, to comply with RFC standards have to have Reverse DNS setup and ISP only do this for fixed IP's with FQDN's - they will add generic rDNS records which will fail checks.
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